It's been hot and muggy in Atlanta the past few days. Now I'm not your typical Atlantan who says that lightly. I grew up in Houston and south Louisiana and spent nine years in Florida, so I know hot and muggy. Overall, summers in Atlanta are pretty nice because of our elevation (~1000 feet), but we do have our moments. Anyway, it's been hot enough that we've hit our summer design temperature, which is defined as the temperature exceeded only 1% of the time. In Atlanta, the summer design temperature is 92 degrees Fahrenheit. That's the temperature at which air conditioners ideally will provide exactly as much cooling as the house needs, when sized according to the Manual J cooling load calculation protocol developed by ACCA, the Air Conditioning Contractors of America. When outdoor conditions are at the design temperature, an air conditioner should run pretty much continuously and be able to keep the house at the ACCA recommended indoor design temperature of 75 degrees Fahrenheit.

When outdoor conditions go above the design temperature, the AC should run continuously and not quite keep the house at 75 degrees. Now, the thing is, Manual J has some built in oversizing, so even if the air conditioner were sized according to an accurately performed Manual J load calculation, it's not going to run continuously at the summer design temperature. my ac unit outside will not turn offIt should run long enough to make the house comfortable, however.air conditioning unit interior Last week I wrote about the 4 factors of comfort, temperature and humidity being the first two. three ton ac unitAn air conditioner actually does two jobs - it lowers the temperature and removes moisture from the air, thus taking care of the first two factors, with HVAC design anyway.

To remove the moisture, however, the air conditioner needs long runtimes. That's because the air moving over the cold evaporator coil causes the water vapor to condense. The more air moves over the coil, the more water condenses out and gets carried away. It takes about 15 minutes of runtime before you start getting serious dehumidification of the air, so oversized systems will not dehumidify well. If the AC comes on, runs 10 minutes or less, and then shuts off, the house may be cool, but in a humid climate, the indoor humidity levels will stay high, probably over 60%. I know this from personal experience. Last year we replaced the AC in our condo, and I did the Manual J load calculation. The result was that we needed a 1.6 ton air conditioner for our 1500 square feet, so we had a 2 ton system installed. Knowing that Manual J has a built-in oversizing bias, I wanted to go with the 1.5 ton system, but I chickened out. Our AC runs maybe 15 minutes max on a hot afternoon, and our relative humidity stays around 60%.

The Manual J bias is real, and it's not small. And that's with an accurate HVAC load calculation. I've seen plenty of load calculations that use incorrect inputs so the contractor can come up with a cooling load to match the AC size he wants to install. Now, back to the point of this article - how to tell if you have an oversized air conditioner. Just get your stop watch and time how long the AC runs on a hot afternoon. Ten minutes or less, and it's definitely oversized. Twenty minutes would be OK. Thirty minutes at a time or longer, and your humidity levels should be fairly low. So, how long does your air conditioner run on a hot day? See my followup article with real data on our AC runtimes: My Big Fat Oversized Air ConditionerThis air-conditioning unit is too big. A 4-ton system installed in the Florida home of Chris Marriner doesn't run long enough to dehumidify the house. Florida is not the kind of place where you'd want to be without air conditioning for very long, so when Chris Marriner's old system died last spring, he didn't waste much time in replacing it.

But what should have been a ticket to indoor comfort hasn't exactly worked out that way. Marriner's HVAC(Heating, ventilation, and air conditioning). technician decided to replace the 4-ton system with one of the same capacity, even though Marriner knew that because of improvements to the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. the new system probably would be oversized for the 2200-square-foot home. The tech told Marriner the system could be "tuned." In a Q&A post at GreenBuildingAdvisor, Marriner complains that indoor relative humidity averages 55% or more, and has ranged up to 66% or 67%. That's apparently because the oversized AC equipment doesn't run long enough to lower indoor humidity. When Marriner spoke to the HVAC installer about the oversized equipment, he got some strange advice.

"I talked to him about it and he wants to bring some attic air into the return so [the AC] will run longer," Marriner writes. "I think that adding a whole house dehumidifier, like the Williams Air Sponge, or Aprilaire would be the best solution. Saving Energy With Manual J and Manual D Green Basics: Cooling Options Q&A: Excess humidity — HVAC system too large Humidity, Mold, and Indoor Air Quality That's the topic for this Q&A Spotlight. Nathaniel G finds the suggestion to bring hot attic air into the system to extend the run time nothing more than an "inefficient Band-aid." The system should have been sized correctly in the beginning. "Sounds like your HVAC tech is less interested in doing his job properly than doing it the way grandpa did and upselling you on a bunch of crap to patch up his own mistakes when grandpa's approach doesn't cut the mustard," Nathaniel says. "Bring some attic air into the return?" writes David Meiland. "You mean deliberately and needlessly increase the load so the unit will run longer in hopes that it will also decrease humidity adequately?

in fact it might be one of the worst ideas I've ever heard — it's about the same as opening a window." It's not clear what the tech might have meant when said the new system could be tuned, Meiland adds, but the real solution is longer run times and an indoor coil that's not so cold. Marriner has already explored the possibility of replacing the outside compressor and scaling back the air-handler. The manufacturer told him that won't work. "To compound this," Marriner adds, the HVAC technician is a friend. "If he were not, I'd have raised hell. He got indignant when I asked him about the humidity and said he will not change the unit ... He was trying to cut me a deal, so I don't know if he had an extra one at the warehouse or what. But I've learned a valuable lesson." senior editor Martin Holladay suggests that Marriner begin with a stand-along dehumidifier. "It may work well enough for you to tolerate your system," he says. "And it is a cheaper solution than starting from scratch."

To help the existing AC equipment get more water out of the air, Holladay says, is to lower the blower speed of the air handler from 400 cubic feet per minute (cfm) per ton to 350 cfm per ton. "Of course, lowering the blower speed (especially below 350 cfm per ton) carries its own risk," he adds, "namely, freezing the coil." Holladay's suggestion for a dehumidifier is a good one, Dana Dorsett says. "If the house is reasonably tight it should do the trick," he writes. "A 600-watt stand-alone dehumidifier converts the latent loadCooling load that results when moisture in the air changes from a vapor to a liquid (condensation). Latent load puts additional demand on cooling systems in hot-humid climates. into a sensible loadThis is the heat content of just the air; it does not include the heat content required to remove (condense) moisture from air. Sensible heat is measured by dry bulb temperature. Latent heat is measured by wet bulb temperature., but it doesn't increase the total cooling load by more than ~2000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match.

/hr (the 600 watts it's using while running). (For more on sensible and latent loads, here's an explanation from a website called The Engineering ToolBox.) "I wonder how many other HVAC technicians there are out there who take the approach of increasing the load as the 'solution' to an oversized AC system?" he adds. "If it's a vented attic, it would also increase the latent load. You may end up pretty much in the same place, but be using significantly more electricity." The indoor humidity levels sound normal, says AJ Builder. "I have no AC and my summer humidity is 50-85% in my home, about 10% less than outside," AJ writes. "No mold except for bad tile work in one shower. Fifty-five seems right considering Tampa, where outside humidity is high along with the temperature." People are healthy and comfortable, Dorsett replies, when the indoor relative humidity (RH) is between 30% and 50% and the temperature between 68° and 75°F. "When it's above 50% RH, dust mite populations begin to grow (a problem primarily for those with allergies or asthma)," Dorsett says.

"Above 60% RH @ 75°F, humans are more susceptible to skin fungus/yeast infections. Above 70% RH it can be pretty uncomfortable, mold spore counts take off, and the risk of respiratory tract infection (fungal or bacterial) goes up." In Tampa, where Marriner lives, high dew points mean that any infiltration of outside air brings in a lot of humidity. "The airtightness of the house and ventilation rates matter quite a bit," he says. AJ isn't buying it: "People who live in a 'clean room' are screwing up their immune systems," he says. "We need to interact with the real world to have healthy immune system understandings IMO and finally in some research published of late. "And Florida, to reduce humidity to 55% (which he has) is fine," AJ adds. "Dehumidify maybe just a bit; but in cooler weather, open windows and be part of this great planet." Like it or not, Dorsett replies, but at 65% humidity "you won't have serious mold issues, but if you have dust mite allergies you're pretty much screwed — they breed faster than flies at that RH."

Moreover, dew points tend to be a lot higher in Tampa than they do in New York (where AJ Builder lives) or in Massachusetts (where Dorsett lives). "That 70°F/55% RH outdoor air you're currently experiencing has a dew point of 53°F, whereas in Tampa right this second it's 92°F, with an outdoor dew point of 72°F (down from the mid-70s from earlier in the day)," Dorsett tells AJ. "Tampa's average outdoor dew point is about the same as our absolute summertime peak dew point so far this year. ( data sets — which was the highest reading of the season.)" Here's how GBA technical director Peter Yost sees it: Higher interior relative humidity can easily feel uncomfortable when you are also dropping the air temperature with air conditioning. Sure, some folks actually do just fine with plenty of air movement at 80°F and 55% RH, but many folks don’t. There is no excuse for any HVAC technician not to size an AC system based on the real loads, or to fail to specify the appropriate sensible heat ratio (SHR – the portion of the total cooling load handling the sensible load) for the climate.